1. Field of the Invention
The present invention relates generally to the production, separation and/or recovery of lactic acid and more particularly to the production, separation and recovery of lactic acid via a fermentation process and the separation and/or recovery of lactic acid from a lactate feed solution such as is obtained from a fermentation broth or other sources.
2. Description of the Prior Art
Lactic acid has long been used as a food additive and in various chemical and pharmaceutical applications. More recently, lactic acid has been used in the making of biodegradable polymers both as a replacement for present plastic materials as well as various new uses where biodegradability is needed or desired. Accordingly, there is an ever increasing demand for lactic acid. The present invention aims at meeting this demand by providing an efficient and environmentally friendly process for producing lactic acid which avoids the consumption of bases and acids and substantially reduces, if not eliminates, the formation of waste or byproduct salts.
Production of lactic acid is commonly carried out by fermentation of a strain of the bacterial genus Lactobacillus and more particularly by the species Lactobacillus delbrueckii or Lactobacillus acidophilus as examples. In general, the production of lactic acid by fermentation in a fermentation broth is well known in the art. The fermentation substrate consists of carbohydrates together with suitable mineral and proteinaceous nutrients. Because the lactic acid producing microorganisms are inhibited in a strongly acidic environment, the pH of the fermentation broth must be kept above 4.5, and preferably within the range of about 5.0 to 7.0, more preferably within the range of about 5.5 to 6.5, and most preferably within the range of about 6.0 to 6.5. To maintain this pH level, suitable water-soluble basic substances or agents that are non-toxic to the acid producing microorganism, such as alkali metal hydroxides, carbonates or bicarbonates or alkaline earth metal hydroxides or carbonates, are commonly added to the fermentation broth to neutralize the acid being produced. This results in the formation of a lactate solution rather than the desired lactic acid product. Such lactate solution contains the lactate anion and the corresponding cation of the substance used to neutralize the fermentation broth.
Various methods have been proposed for the recovery of lactic acid from a fermentation broth. Where the fermentation is carried out in the presence of calcium carbonate, it is possible to recover the lactic acid by acidification with sulfuric acid. This results in the precipitation of calcium sulfate, while free lactic acid remains in the mother liquor. If desired, the mother liquor may be concentrated to up to about 90% by weight lactic acid. Subsequently, lactic acid may be extracted from the mother liquor with a suitable organic extractant to yield an extract which is back-extracted with water or the acid may be adsorbed on a suitable adsorbent and later desorbed. The resulting aqueous lactic acid solution may then be concentrated. This method has the disadvantage that it irreversibly consumes calcium carbonate and sulfuric acid and leaves, as waste, large quantifies of calcium sulfate which give rise to disposal problems.
U.S. Pat. No. 5,132,456 (King et al.) describes a process for recovering carboxylic acid from a carboxylic acid-containing aqueous feed stream having a pH close to or above the pK.sub.a level of the acid. In accordance with that process the recovery involves what may be described as a cascade type acid withdrawal operation in which the basicity of the extractant is increased stepwise. In a first stage of the process, the feed stream is contacted with an adsorbent such as a strongly basic extractant or a solid anion exchanger. In a second stage the acid-loaded adsorbent is contacted with an aqueous solution of ammonia or a low molecular weight trialkyl amine having a stronger affinity to the carboxylic acid that is being recovered than the adsorber used in the first stage. In this way an aqueous solution of a water-soluble carboxylic acid ammonium salt is formed. This is then subjected to heat treatment, whereby the salt is decomposed to yield back the trialkyl amine or ammonia and free carboxylic acid. Applying this process to lactic acid involves the formation of salts of lactic acid with strong bases having a pK.sub.a value of about 9-11. Thus, the decomposition of these salts into free lactic acid is energy intensive. Examples 12-14 of the patent mention the use of Alamine 336 (tricaprylylamine) for the extraction of, among others, lactic acid from an aqueous solution, but no yields are mentioned. By the extraction of even small quantities of lactic acid from a fermentation broth the pH of the broth rises rapidly to above 7. As shown in FIGS. 3 and 4 of this patent, the uptake of carboxylic acids from aqueous solutions drops rapidly with an increase of the pH. It is, therefore, inherent in these examples that the lactic acid uptake, if any, is negligible. It is further noted that upon heat treatment and concentration of an ammonium lactate, crystalline lactic acid does not precipitate and instead the viscosity of the solutions increases steadily as a result of self-association of the acid. It is thus evident that the process of U.S. Pat. No. 5,132,456 is unsuitable for the recovery of lactic acid from a fermentation broth.
U.S. Pat. Nos. 4,444,881 and 4,405,717 (Urbas) describe a process for the recovery of an organic acid from a diluted aqueous solution of its calcium salt by adding a water-soluble trialkyl amine carbonate to the solution to form on the one hand a water soluble trialkyl ammonium salt of the acid, which salt remains in solution, and on the other hand calcium carbonate which precipitates. After removal of the calcium carbonate the remaining mother liquor is heated for the separate recovery of the amine and the product acid. The water-soluble trialkyl amines employed in accordance with these patents are strongly basic. Accordingly, the decomposition of the trialkylammonium salts into free acids is energy intensive.
U.S. Pat. No. 4,282,323 (Yates) describes a process for obtaining lower carboxylic acids from a salt solution of such carboxylic acid as obtained from fermentation. The process appears to be applicable to a restricted number of lower aliphatic and aromatic monocarboxylic acids and is specifically described only in relation to acetic acid. In accordance with that process, the aqueous solution of a carboxylic acid salt is contacted in the presence of a liquid polar organic solvent serving as extractant, with pressurized carbon dioxide, to convert at least part of the salt to the corresponding free acid which is taken up by the organic phase from where it is subsequently recovered. It is inherent in the use of a polar organic extractant that the bulk of the carboxylic acid remains in the neutral to basic aqueous phase, and indeed the recovery rates reported in U.S. Pat. No. 4,282,323 are low, ranging between 4.8% and 18% of the acid initially present.
U.S. Pat. No. 4,275,234 (Baniel) is directed to a method of recovering various acids in their free form from aqueous solutions. Thus, the process of Baniel is not applicable to a lactate solution of the type commonly obtained from a fermentation process or from other sources. The essence of the Baniel U.S. Pat. No. 4,275,234 is the discovery that efficient back-extraction can be achieved by performing the back-extraction at a temperature higher than that of the primary extraction.
R. Bar and J. L. Geiner, Biotechnology Progress 3, 109 (1987) studied the feasibility of extracting lactic acid from aqueous solution by means of a long-chain trialkyl amine of low basicity, such as tridodecylamine, using various tridodecylamine solutions in n-dodecanol. It was found that extraction of lactic acid by extraction with a long-chain trialkyl amine such as tridodecylamine was effective only at a pH that is lower than the pK.sub.a of lactic acid, the latter of which is 3.86. At such a low pH, however, the lactic acid fermenting microorganism such as, for example, Lactobacillus delbrueckii or Lactobacillus acidophilus is severely inhibited.
To sum up, the prior art teaches that when using amines for the recovery of lactic acid from a lactate solution such as that obtained from a fermentation broth, either by way of extraction or by way of ammonuim salt formation, only strongly basic water soluble amines can be used, while water immiscible amines of weak basicity, such as long-chain trialkyl amines, are unsuitable. Accordingly, a need exists for a lactic acid production process which is energy efficient, is environmentally friendly, consumes no acids or bases and which minimizes, if not eliminates, generation of waste salts or emissions.